The present invention relates to a control circuit for a thermal conductivity cell and particularly one which provides linearity, lower noise, and filament protection.
A variety of constant resistance thermal conductivity detector circuits exist which employ a single filament in the leg of a Wheatstone bridge. A servo loop forces the bridge to balance, thereby holding the filament at a constant resistance. With such a system, however, the single filament is subject to significant drift since there is no reference filament and the output is sensitive to the temperature of the thermal conductivity cell block. Further, the signal can suffer from non-linearities inasmuch as the thermal conductivity is proportional to the power dissipated and the voltage across the filament. Sensitivity suffers inasmuch as a high common mode output signal to drive the bridge is necessary, and, therefore, gain cannot be applied to the output signal to increase the sensitivity of the cell. Finally, without a reference filament, the effects due to vibrations and pressure variations are apparent in the output signal.
Attempts have been made to control drift by altering the filament resistance between two temperatures resulting in a differential signal independent of block temperature. Other approaches have employed a bridge control signal alternated between constant resistance and constant voltage resulting in a similar differential control signal. In some approaches, drift has been compensated for by attaching a temperature sensor to the cell block itself. The linearity problem has also been addressed utilizing a closed-loop analog circuit in most cases. In some instances, a resistance loop is closed employing a digital signal and comparator to sense an imbalance in the microprocessor controlled D-to-A converter to drive the bridge back to balance. These configurations have the advantage of having a linear output verses concentration as the pulse width is directly related to power dissipated in the filament.
The sensitivity of a detection system utilizing a thermal conductivity cell has been addressed by various analog methods, typically including the utilization of a voltage divider and switch capacitor network to reference the voltage across each resistor to a common ground. The sensitivity of such a system is somewhat improved over other methods, however, the sensitivity is still limited to about 2 ppm (parts per million) detection. The problem with pressure disturbances in a thermal conductivity system has been addressed as well in which two measurement filaments and two reference filaments have been employed in a four filament bridge in an effort to control the average resistance of the bridge to compensate for such pressure disturbances.
Thus, although the prior art has attempted to address individually each of the various problems inherent in a thermal conductivity detection system, the prior art has not solved each of the problems adequately nor comprehensively addressed these problems in an overall system which provides improved linearity, low noise, and a protected thermal conductivity system in which the thermal conductivity resistance filament is protected from oxidation. There exists, therefore, a need for an improved thermal conductivity control circuit which provides these advantages.
The system of the present invention provides a control circuit for a thermal conductivity cell by employing a constant resistance bridge drive circuit which automatically adjusts to accommodate various carrier gases, such as helium and argon. Additionally, a reference filament is placed in a third leg of the bridge to provide a differential measurement signal. The detection circuit utilizes digital/analog methods to significantly reduce 1/f noise of an amplifier providing at least a seven fold improvement in signal-to-noise ratio. This also eliminates the thermocouple effects in the path from the bridge to the amplifier.
The circuit includes a bridge nulling method which is adjusted under microprocessor control to eliminate the need for manual offset adjustments. Thus, the typical necessity of matching the measurement and reference filaments can be relaxed, thereby reducing the cost of a thermal conductivity cell employing such a control circuit. The automatic monitoring of the bridge filament can warn the operator when filaments have aged and require replacement. The reference filament is monitored individually and protected against exceeding its oxidation temperature. This system compensates for fluctuation in cell block temperature, exhaust pressure variations, and improves the linearity more than ten fold over conventional detection bridges.
These features and advantages of the present invention are embodied in a system in which reference and measurement filaments are incorporated in a Wheatstone bridge and a constant resistance circuit compares the bridge drive voltage applied to the bridge divided in half in a closed-loop feedback circuit to maintain the measurement filament a constant resistance. Signals from the reference detector are modulated at a frequency above the significant 1/f noise frequency of an amplifier in a preferred embodiment of the invention, amplified, demodulated, and filtered to provide a low noise output signal representative of the concentration of a sample gas through the measurement cell.
According to another aspect of the invention, the reference filament drive is continually adjusted for changes in relative resistance between the reference filament and the measurement filament by a null adjustment circuit coupled to the reference filament and to the bridge drive circuit. Output signals are coupled to a microprocessor for controlling the null adjustment to null any differences between the resistive legs of the bridge prior to an analysis. According to another aspect of the present invention, a reference protection circuit is provided and is coupled to the reference filament and controls the voltage applied to the filament through a voltage divider and integrator circuit to prevent overheating of the reference electrode in the event there is a breach in the gas flow path or other event which otherwise would allow the filament to overheat.